1. Field of the Invention
The present invention relates to an optical pick-up apparatus that is a key component of an optical disc drive apparatus. More particularly, the present invention relates to an optical pick-up apparatus for recording, reproducing and erasing information with respect to both of a group of discs including CD, CD-ROM, CD-R and CD-RW (hereinafter called “CD group disc”) and a group of discs including DVD, DVD-ROM, DVD-RAM, DVD-R and DVD-RW (hereinafter called “DVD group disc”), which have different track pitches from each other.
2. Related Background Art
A configuration and an operation of a conventional optical pick-up apparatus will be described below, with reference to FIG. 3. This apparatus enables information to be recorded, reproduced and erased with respect to a CD group disc 110 and a DVD group disc 111. Although FIG. 3 shows both of the CD group disc 110 and the DVD group disc 111 for the purpose of illustration, as a practical matter, only one of these discs is loaded thereon. The CD group disc 110 or the DVD group disc 111 is placed on a common plane P facing an objective lens 109. On the reverse side of the plane P, recording surfaces 110a and 111a are provided. Since the CD group disc 110 has a thickness larger than that of the DVD group disc 111, the recording surface 110a of the CD group disc 110 is positioned further from the objective lens 109 than the recording surface 111a of the DVD group disc 111.
This apparatus has an infrared semiconductor laser device 101 and a red semiconductor laser device 102. In optical paths between the laser devices 101 and 102 and the discs 110 and 111, 3-beam generating diffraction gratings 103 and 104, a beam splitter 105, a collimator lens 106, a beam splitter 107, a condenser lens 112, a photodetector 113, a mirror 108 and the objective lens 109 are arranged. The 3-beam generating diffraction gratings 103 and 104 form three beams from optical beams emitted from the infrared semiconductor laser device 101 and the red semiconductor laser device 102, respectively. The beam splitter 105 guides the beams that have passed through the 3-beam generating diffraction gratings 103 and 104 onto the common optical axis. The collimator lens 106, the objective lens 109 and the condenser lens 112 function as a light-gathering device. The beam splitter 107 divides the reflected light from the disc 110 or 111.
This optical pick-up apparatus operates as follows: when recording, reproducing or erasing information with respect to the CD group disc 110, the infrared semiconductor laser device 101 operates. An optical beam emitted from the infrared semiconductor laser device 101, which is indicated by a solid line, is diffracted by the 3-beam generating diffraction grating 103 so as to be divided into three optical beams (a main beam as zero-order diffracted light and side beams as±first-order diffracted light). These three optical beams pass through the beam splitter 105, then are converted from the divergent beams to parallel beams by the collimator lens 106, and enter into the objective lens 109 via the mirror 108 to be focused onto the CD group disc 110. Reflected light from the CD group disc 110 passes through the objective lens 109 and the mirror 108, is directed to a different direction by the beam splitter 107, and is focused onto the photodetector 113 by the condenser lens 112. From the main beam and side beams incident on the photodetector 113, an RF signal, a focus error signal and a tracking error signal are detected.
Meanwhile, when recording, reproducing or erasing information with respect to the DVD group disc 111, the red semiconductor laser device 102 operates. An optical beam emitted from the red semiconductor laser device 102, which is indicated by a dashed line, is diffracted by the 3-beam generating diffraction grating 104 so as to be divided into three optical beams (a main beam as zero-order diffracted light and side beams as±first-order diffracted light). These three optical beams are directed to a different direction by the beam splitter 105, then are converted from the divergent beams to parallel beams by the collimator lens 106, and enter into the objective lens 109 via the mirror 108 to be focused onto the DVD group disc 111. Reflected light from the DVD group disc 111 passes through the objective lens 109 and the mirror 108, is directed to a different direction by the beam splitter 107, and is focused onto the photodetector 113 by the condenser lens 112. From the main beam and side beams incident on the photodetector 113, an RF signal, a focus error signal and a tracking error signal are detected.
The conventional optical pick-up apparatus shown in FIG. 3, however, requires the two semiconductor laser devices, the two diffraction gratings and the beam splitter 105 for guiding the emitted light from the two semiconductor laser devices to the common optical axis in order to be compatible with the CD group disc 110 and the DVD group disc 111. Therefore, it is difficult to downsize the optical pick-up apparatus. In addition, since a large number of optical components are required, which complicates the positional adjustment among the respective optical components, the cost required for components and for adjusting the components increases significantly as compared with an optical pick-up apparatus dealing with a single type of disc.